well, i've known for as long as i can remember. i'm not sure how i found out, but i knew before we talked about it in school. i never had any problem thinking that i didn't have to worry about getting a girl pregnant because my dad was sure to instill proper sexual ethics.
maybe seven is a bit young, but i wouldn't wait too long. i think we often underestimate kids and have this overwhelming urge to protect them that may cause more harm than good.
when he is old enough to pleasure himslef, which is something that boys discover fairly early, he might notice that something is different than what he may have already heard or have been taught in school. if he is comfortable and open, he might ask questions then, but i wouldn't count it. i put my money on confusion and maybe some apprehension.
even though knew i was infertile, i wasn't prepared for the lack 'substance' upon climax, which caused me a fair amount of anxiety and apprehension towards sex, even today. it tool me over twenty years to be able to talk about it seriously.